A simple, economical, macrophage migration inhibition assay enabling multiple tests to be performed using microculture plates has been developed. The system confers many advantages over the standard in vitro assay of macrophage migration inhibition factor. It requires only 5 X 10(5) macrophages for each assay, small volumes (100 microliter) of test factors are consumed, 96 tests in each culture plate are easily assessed microscopically and the technique is easily and reproducibly executed. An adherent monolayer of activated fresh mouse peritoneal exudate cells (PEC) is established in each microculture well. A 'starting line' is scored across the monolayer by the application of a trimmed razor blade and the cells are dislodged and aspirated from one side of the line. Test factors from cultures of stimulated lymphocytes were added to the 'wounded' monolayer and the migration of macrophages from test and control cultures was measured after staining. The cells across the line were counted after incubation. Miration was shown to be time and temperature dependent, markedly stimulated by colchicine and appeared to associate with tuberculin antigen (PPD) skin testing and lymphocyte transformation in human subjects.